1. Field of the Invention
The present invention relates in general to a braking system for automotive vehicles such as four-wheel automobiles, two-wheel motor cycles, or various vehicles or trucks for industrial purposes.
2. Discussion of the Prior Art
Hydraulically operated braking systems have long been used for automotive vehicles. Such a hydraulically operated braking system includes an operator-controlled brake operating member, a master cylinder responsive to the operation of the brake operating member, for generating a fluid pressure corresponding to an operating force acting on the brake operating member, and brakes actuated by the fluid pressure developed by the master cylinder, in order to apply brakes to the wheels of a vehicle. The braking effect produced by the brakes can be adjusted by the vehicle operator, by controlling the operating force exerted on the brake operating member.
In the hydraulic braking system of the type indicated above, the actual braking effect may be varied due to changes in the friction coefficient of the brake pads and in the load condition of the vehicle, even if the operating force on the brake operating member is kept constant. Also, it is difficult to precisely or intricately control the operating force acting on the brake operating member. Accordingly, the braking effect may be fluctuated as a function of time during brake application by a single operation of the brake operating member. Thus, the conventional hydraulic braking system suffers from the drawbacks discussed above.
In view of the above drawbacks, provisions for automatic intricate control of the fluid pressure in the master cylinder have been proposed in recent years. An example of a braking system incorporating such provisions is disclosed in Laid-Open Publication No. 58-188746 of Japanese Patent Application, wherein a booster of a hydraulic pressure type is disposed between the brake operating member and the master cylinder. The boosting ratio of the booster is automatically controlled to precisely regulate the fluid pressure to be generated by the master cylinder, so that the desired braking effect can be obtained under any conditions.
In the proposed system indicated above wherein the level of the fluid pressure generated by the master cylinder is regulated, it is required that the fluid pressures in the brakes connected to the master cylinder be changed over a wide range, from a low level used for light braking, up to a high level required for rapid and heavy braking. A braking system capable of producing a suitably controlled braking effect over such a wide range of the brake pressure is difficult to design and manufacture.
It will be understood that the amount or force of operation of the brake operating member by the vehicle driver substantially corresponds to a magnitude of a braking effect desired by the vehicle driver. Therefore, although the actual braking effect obtained does not agree to the desired or optimum braking effect, the amount of deviation of the actual braking effect from the desired braking effect is usually not so large. In light of this relatively small deviation, it is considered to provide a braking system which has a second brake, in addition to a first brake which is actuated to provide a braking effect corresponding to the amount or force of operation of the brake operating member by the vehicle driver. According to this arrangement, the first brake is designed so that the braking effect produced is slightly smaller than the desired or ideal braking effect, while the second brake is used to provide a braking effect which is equal to a difference between the braking effect produced by the first brake and the desired braking effect. In this case, a controller is used only for the second brake, for regulating the braking effect to be produced by the second brake. Thus, the required range of control of the controller is comparatively narrow.
The above concept of using a second brake assigned to compensate for a shortage of braking effect produced by a first brake has not been found in the prior art, except a similar type of arrangement as disclosed in Laid-Open Publication No. 57-51562 of Japanese Patent Application, wherein a fluid type retarder is used as a second brake, in addition to a hydraulically operated friction brake as a first brake. In this arrangement, the retarder is used to compensate for a shortage of the braking effect of the first brake with respect to the desired braking effect. However, the braking effect to be produced by such a retarder as a special brake cannot be controlled by the vehicle operator. Further, the retarder provides substantially no braking effect while the vehicle is running at a low speed. In this arrangement, too, therefore, the friction brake as the first brake should provide a braking effect which is variable over a wide range. An anti-skid braking system is disclosed in U.S. Pat. No. 4,508,393, wherein two brake cylinders are provided for a single disc rotor of a wheel. The pressures in the two brake cylinders are controlled independently of each other, so as to prevent skidding of the wheel on the road surface. In this arrangement, however, the fluid pressures in the two brake cylinders are not controlled as a function of the amount or force of operation of the brake operating member by the vehicle operator, while the fluid pressures are automatically controlled by a controller of the braking system. Hence, the controller of this anti-skid braking system is also required to be capable of controlling the fluid pressures over a wide range.